The Causes and Implications of Microstructures in Glacial Sediments

This thesis examines how microstructures in glaciogenic sediments reflect the processes
forming them, and how these microstructures then affect the conditions around them, through a
series of field studies, laboratory tests, models, and statistical analyses. Following literature
reviews, a deformational chronology is developed for diamictons at Criccieth, North Wales,
and their microstructures are used to indicate the stress, hydraulic, and environmental changes
the materials have undergone. Microstructures of the lowest diamict indicate clast lodgement.
The processes reflected in the microstructures of this lowest diamict are built into a quantitative
model that estimates its residual strength (20 - 50 kPa) and the ice velocity during lodgement
(20 - 50 m a-1). The response of sediment to glacial stress is further examined by triaxial
testing of diamict from Yorkshire, and the subsequent examination of its micromorphology.
Shears in the material are disrupted by clasts, and this may be responsible for work hardening
seen during the tests. Fabric compression, and the development of immobilised shears or
hydraulic fractures buffer pore fluid pressure to ~470 kPa. The information from previous
chapters is then used to analyse other material from the Yorkshire coast. This analysis confirms
the presence in the area of meltout tills that have undergone low strain, as well as providing
evidence for the decoupling of the ice and sediment in this region, and the nature of drainage
systems within and above the diamicts during glaciation. Overall this thesis details the
processes forming three ‘classic’ microstructures found in glacial sediments; omnisepic fabrics,
lattisepic fabrics, and melanges, and provides evidence for the processes involved in forming
diamict pebbles and skelsepic fabrics. In addition this thesis details how such structures reflect
coupling and decoupling processes between glaciers and their beds, and examines the manner
in which microstructures affect the response of a subglacial sediment body to stress and
hydraulic conditions.